1. Field of the Invention
The present invention relates to an ultraviolet-ray radiation type high intensity discharge lamp (metal halide lamp) used in curing a printing ink or resist ink utilizing a photochemical reaction.
2. Description of the Related Art
A metal halide lamp in which iron and a halogen together with a starting rare gas and mercury are enclosed in an arc tube having electrodes at its both ends has a good light emission efficiency of ultraviolet rays especially in a UV-A range (315 to 400 nm). Therefore, the metal halide lamp is often used as a light source for a photochemical reaction such as a curing light source for a printing ink, instead of a mercury lamp. However, as an operating time passes, a black deposit is deposited on the inner wall of an arc tube of this lamp to largely reduce ultraviolet-ray intensity.
In order to eliminate the above drawback, Japanese Patent Disclosure (Kokai) No. 57-63757 or 57-101329 discloses a method of suppressing the tube-wall-blackening by adding palladium, zirconium, or titanium in an arc tube.
In a technique disclosed in U.S. Pat. No. 3,590,307, an amount of iron to be enclosed in an arc tube is set to be 0.01 to 1 mg/cc, a halogen (iodine) is enclosed in the arc tube in an amount sufficient to form iron iodide (FeI.sub.2) together with iron, and then a halide of tin (SnI.sub.4) is added. In this case, ratio {[Fe]+[Sn]}/[J ] of total gram atoms of iron (Fe) and tin (Sn) to gram atoms of halogen (J) is 0.3 to 0.5.
Japanese Patent Publication No. 58-18743 discloses a metal vapor discharge lamp aiming at improving a spectral distribution. In this lamp, an enclosing amount of a halogen is 1.0.times.10.sup.-8 to 1.0.times.10.sup.-5 gram atom/cc, a ratio of total gram atoms of iron and tin to gram atoms of the halogen is 0.5 to 3, and a gram atom ratio of tin to iron is 0.05 to 3.
However, even the above lamp does not have a sufficient service life to be used as a curing light source for a printing ink (as will be described later). In addition, since a processing time of a printer is becoming shorter and shorter, the ultraviolet-ray intensity must be sufficiently high in order to sufficiently cure an ink within a short radiation time.